Optical communication system and optical amplifier

ABSTRACT

An optical communication system having a transmitting station for outputting WDM (wavelength-division multiplexing) signal light, an optical fiber transmission line, a receiving station, and an optical repeater including an optical amplifier. The transmitting station includes a supervisory circuit for detecting the number of channels of the WDM signal light and transmitting supervisory information including the number of channels to the optical repeater. The optical repeater further includes a circuit for controlling the optical amplifier so that the output level of the optical amplifier becomes a target level. The target level is set according to the supervisory information. According to the structure, it can be possible to provide a system which can easily respond to a change in the number of WDM channels.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to an opticalcommunication system and an optical amplifier suitable for long-haul andlarge-capacity transmission, and more particularly to an opticalcommunication system applicable to WDM (wavelength-divisionmultiplexing) and an optical amplifier suitable for WDM.

[0003] 2. Description of the Related Art

[0004] In recent years, research and development on application of anoptical amplifier to an optical communication system have beenintensively pursued. For example, importance of a booster amplifier,optical repeater, and preamplifier having an EDFA (erbium doped fiberamplifier) has become apparent.

[0005] Conventionally known is an optical amplifier comprising anoptical amplifying medium for amplifying signal light and means forpumping the optical amplifying medium so that the optical amplifyingmedium has an amplification band including the wavelength of the signallight. In the case that the optical amplifying medium is an EDF (erbiumdoped fiber) having a first end and a second end, the pumping meansincludes a pump light source for outputting pump light having a properwavelength, and means for supplying the pump light into the doped fiberfrom at least one of the first end and the second end. In the case thatthe optical amplifying medium is provided by a semiconductor chip, thepumping means includes means for injecting a current into the chip.

[0006] To greatly increase a transmission capacity, a WDM system(wavelength-division multiplexing system) has been proposed. The WDMsystem includes a first terminal station for outputting WDM signal light(wavelength-division multiplexed signal light) obtained bywavelength-division multiplexing a plurality of optical signals havingdifferent wavelengths, an optical transmission line for transmitting theWDM signal light output from the first terminal station, and a secondterminal station for receiving the WDM signal light transmitted throughthe optical transmission line. To increase a transmission distance inthe WDM system, one or more optical repeaters each having an opticalamplifier are provided in the optical transmission line.

[0007] In applying the optical amplifier to the WDM system, gain tiltoccurring in the optical amplifier must be considered. The gain tilt isbased on the wavelength dependence of gain. In an EDFA, for example, thegain tilt changes with a change in total input power because ofcharacteristics of homogenous broadening of an EDF. Accordingly, inoperating the WDM system or the optical repeater, it is desirable tograsp the gain tilt of the optical amplifier and maintain a constantgain tilt.

[0008] In the optical amplifier or the optical repeater, a feedback loopfor ALC (automatic level control) is usually adopted, so as to maintainthe output level constant. In applying the optical amplifier adoptingALC to the WDM system, a target level in ALC for maintaining outputpower per channel constant changes with a change in the number ofchannels of WDM signal light. Accordingly, the conventional opticalcommunication system cannot easily respond to a change in the number ofchannels.

SUMMARY OF THE INVENTION

[0009] It is therefore an object of the present invention to provide anoptical communication system which can easily respond to a change in thenumber of channels in WDM.

[0010] It is another object of the present invention to provide anoptical amplifier which can maintain a constant gain tilt.

[0011] It is a further object of the present invention to provide anoptical amplifier which can maintain a constant gain tilt and allowsautomatic level control.

[0012] An optical communication system to which the present invention isapplicable includes first and second terminal stations, an opticaltransmission line connecting the first and second terminal stations, andan optical repeater provided in the optical transmission line. The firstterminal station includes a plurality of optical transmitters foroutputting optical signals having different wavelengths, and a means forwavelength-division multiplexing the optical signals to output WDMsignal light. The WDM signal light is transmitted by the opticaltransmission line, and received by the second terminal station. Theoptical repeater includes an optical amplifier for amplifying the WDMsignal light.

[0013] In accordance with a first aspect of the present invention, thefirst terminal station further includes a means for detecting the numberof channels of the WDM signal light, and a means for transmittingsupervisory information indicating the number of channels to the opticalrepeater. The optical repeater further includes a means for detecting anoutput level of the optical amplifier, and a means for controlling theoptical amplifier so that the output level detected becomes a targetlevel. In the optical repeater, for example, the target level is setaccording to the supervisory information transmitted from the firstterminal station.

[0014] In accordance with a second aspect of the present invention, theoptical repeater further includes a means for detecting an output levelof the optical amplifier, and a means for controlling the opticalamplifier so that the output level detected becomes a target level. Thetarget level is constant irrespective of the number of the opticaltransmitters being operated. Preferably, the optical signals to beoutput from some of the plurality of optical transmitters being operatedare modulated by main signals, and the optical signals to be output fromthe other optical transmitters not being operated are continuous waves.

[0015] In accordance with a third aspect of the present invention, theoptical amplifier includes an optical amplifying medium, a means forpumping the optical amplifying medium so that the optical amplifyingmedium has an amplification band including the wavelengths of the WDMsignal light, a light source for outputting compensation light having awavelength included in the amplification band but different from thewavelengths of the WDM signal light, and a means for supplying the WDMsignal light and the compensation light to the optical amplifyingmedium. The optical repeater further includes a means for detecting anoutput level of the optical amplifier, a means for controlling theoptical amplifier so that the output level detected becomes a targetlevel, and a means for controlling power of the compensation light sothat the target level becomes constant irrespective of the number ofchannels of the WDM signal light.

[0016] In accordance with a fourth aspect of the present invention,there is provided an optical amplifier comprising an optical amplifyingmedium having a first end and a second end, the first end receivingsignal light; a first means for pumping the optical amplifying medium sothat the optical amplifying medium has an amplification band including awavelength of the signal light; a second means operatively connected tothe first end of the optical amplifying medium, for monitoring spectralcharacteristics of amplified spontaneous emission propagating in adirection opposite to a propagation direction of the signal light in theoptical amplifying medium; and a third means for controlling a gain inthe amplification band so that the spectral characteristics aremaintained.

[0017] In accordance with a fifth aspect of the present invention, thereis provided an optical amplifier comprising an optical amplifying mediumhaving an optical waveguide structure into which signal light issupplied; a means for pumping the optical amplifying medium so that theoptical amplifying medium has an amplification band including awavelength of the signal light; a means for extracting spontaneousemission leaked sideways from the optical waveguide structure; a meansfor monitoring spectral characteristics of the spontaneous emission; anda means for controlling a gain in the amplification band so that thespectral characteristics are maintained.

[0018] In accordance with a sixth aspect of the present invention, thereis provided an optical amplifier comprising first and second opticalamplifier units and a means for cascading the first and second opticalamplifier units. Each of the first and second optical amplifier unitshas the configuration of the optical amplifier in accordance with thefourth aspect of the present invention. This optical amplifier furthercomprises an optical attenuator having a variable attenuation factor,for attenuating amplified signal light output from the first opticalamplifier unit; a means for branching amplified signal light output fromthe second optical amplifier unit into first branch light and secondbranch light; a photodetector for receiving the first branch light; anda means for controlling the attenuation factor of the optical attenuatorso that an output level of the photodetector becomes constant.

[0019] The above and other objects, features and advantages of thepresent invention and the manner of realizing them will become moreapparent, and the invention itself will best be understood from a studyof the following description and appended claims with reference to theattached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a block diagram of a WDM system to which the presentinvention is applicable;

[0021]FIG. 2 is a block diagram showing a first preferred embodiment ofan optical repeater;

[0022]FIG. 3 is a block diagram of an ALC circuit;

[0023]FIG. 3A is a block diagram showing a concrete example of theoptical repeater shown in FIG. 2;

[0024]FIG. 4 is a block diagram of another WDM system to which thepresent invention is applicable;

[0025]FIG. 5 is a graph for illustrating a tone signal;

[0026]FIG. 6 is a block diagram showing a second preferred embodiment ofthe optical repeater;

[0027]FIG. 7 is a block diagram of still another WDM system to which thepresent invention is applicable;

[0028]FIG. 8 is a block diagram showing a third preferred embodiment ofthe optical repeater;

[0029]FIG. 9 is a block diagram showing a fourth preferred embodiment ofthe optical repeater;

[0030]FIG. 10 is a block diagram showing a first basic configuration ofan optical amplifier;

[0031]FIGS. 11A and 11B are graphs for illustrating preferredembodiments for removing the influence of accumulated ASE;

[0032]FIG. 12 is a block diagram showing a first preferred embodiment ofthe optical amplifier;

[0033]FIG. 13 is a graph for illustrating a gain tilt;

[0034]FIG. 14 is a block diagram of a spectrum monitor;

[0035]FIG. 15 is a block diagram of another spectrum monitor;

[0036]FIG. 15A is a block diagram showing a concrete example of theoptical amplifier shown in FIG. 12;

[0037]FIG. 15B is a block diagram showing another concrete example ofthe optical amplifier shown in FIG. 12;

[0038]FIG. 16 is a block diagram showing a second preferred embodimentof the optical amplifier;

[0039]FIG. 17 is a block diagram showing a third preferred embodiment ofthe optical amplifier;

[0040]FIG. 18 is a block diagram showing a fourth preferred embodimentof the optical amplifier;

[0041]FIG. 19 is a block diagram showing a second basic configuration ofthe optical amplifier;

[0042]FIG. 20 is a block diagram of a spectrum monitor that can be usedin the second basic configuration shown in FIG. 19; and

[0043]FIG. 21 is a block diagram showing a third basic configuration ofthe optical amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Some preferred embodiments of the present invention will now bedescribed in detail with reference to the attached drawings.

[0045]FIG. 1 is a block diagram of a WDM system to which the presentinvention is applicable. This system includes a transmitting station 2for outputting WDM signal light, an optical fiber transmission line 4for transmitting the WDM signal light output from the transmittingstation 2, and a receiving station 6 for receiving the WDM signal lighttransmitted through the optical fiber transmission line 4.

[0046] The transmitting station 2 has a plurality of opticaltransmitters 8 (#1 to #5). Each optical transmitter 8 has a terminal 10for receiving a drive signal. Each optical transmitter 8 outputs signallight(optical signal) having a predetermined wavelength given by directmodulation of a laser diode or by modulation of CW light (continuouswave light) from a laser diode by an optical modulator. Status signals12 from the optical transmitters 8 (#1 to #4) are supplied to a SVcircuit (supervisory circuit) 14. Each status signal 12 includes a flagrepresenting whether or not the corresponding optical transmitter 8 isin operation. The SV circuit 14 outputs supervisory informationincluding the number of the optical transmitters 8 being operated, thatis, the number of channels of WDM signal light. The supervisoryinformation from the SV circuit 14 is input to the terminal 10 of theoptical transmitter 5 (#5), and an optical signal having a wavelengthλ_(SV) modulated by the supervisory information is output from theoptical transmitter 8 (#5). The optical transmitters 8 (#1 to #4) outputoptical signals having wavelengths λ₁ to λ₄ modulated by transmissiondata (main signals) of the respective channels. The optical signals fromall the optical transmitters 8 are combined together by a multiplexer(MUX) 16, and the WDM signal light thus obtained is then output to theoptical transmission line 4.

[0047] Two optical repeaters 18 are provided in the optical transmissionline 4. Three or more optical repeaters 18 may be provided or oneoptical repeater 18 may be provided. Each optical repeater 18 has anoptical amplifier 20 for amplifying the WDM signal light and outputtingamplified WDM signal light, and a SV circuit (supervisory circuit) 22for inputting/outputting the supervisory information transmitted fromthe transmitting station 2 from/to the optical amplifier 20. Eachoptical repeater 18 further has an optical coupler 24 to supply theoptical signal having the wavelength λ_(SV) modulated by the supervisoryinformation in such a manner as to bypass the optical amplifier 20, sothat an updated optical signal (having the wavelength λ_(SV)) outputfrom the SV circuit 22 is joined to the optical fiber transmission line4 by an optical coupler 26.

[0048] In the present application, the term of “an optical repeaterprovided in an optical transmission line” may be used as arepresentative of optical amplifiers such as an optical post-amplifier,an optical pre-amplifier, and so on.

[0049] The receiving station 6 has a demultiplexer (DEMUX) 28 forbranching the WDM signal light transmitted by the optical fibertransmission line 4, and a plurality of optical receivers 30 (#1 to #4)for demodulating the transmission data of all the channels according tothe WDM signal light branched. The transmission data demodulated by eachoptical receiver 30 is output from a terminal 32 of the correspondingoptical receiver 30.

[0050]FIG. 2 is a block diagram showing a first preferred embodiment ofthe optical repeater. This optical repeater may be used as each opticalrepeater 18 shown in FIG. 1.

[0051] Throughout the drawings, substantially the same parts are denotedby the same reference numerals.

[0052] The signal light having the wavelength λ_(SV) branched off fromthe optical fiber transmission line 4 by the optical coupler 24 ispreviously modulated by the supervisory information. This signal lightis input to an O/E converter (opto-electric converter) 34 incorporatedin the SV circuit 22, and the supervisory information is regeneratedaccording to an output signal from the O/E converter 34. The WDM signallight having the wavelengths λ_(j)(j=1 to 4) passed through the opticalcoupler 24 is branched into first branch light and second branch lightby an optical coupler 36. The first branch light is supplied to anoptical amplifying medium 38. The optical amplifying medium 38 ispreviously pumped by pumping means 40. Accordingly, the opticalamplifying medium 38 has an amplification band including the wavelengthsof the WDM signal light. As the optical amplifying medium 38, a dopedfiber doped with a rare earth element, e.g., an erbium doped fiber (EDF)may be used. In this case, the pumping means 40 includes a pump lightsource operatively connected to at least one of a first end and a secondend of the doped fiber, for supplying pump light to the doped fiber.Alternatively, a semiconductor chip may be used as the opticalamplifying medium 38 (semiconductor optical amplifier). In this case,the pumping means 40 includes means for applying a pumping voltageacross a pair of electrodes provided in the chip to inject a current. Inthe following description, it is assumed that the optical amplifyingmedium 38 is a doped fiber and the pumping means 40 includes a pumplight source.

[0053] The signal light amplified by the optical amplifying medium 38 isbranched into first branch light and second branch light by an opticalcoupler 42. The first branch light from the optical coupler 42 istransmitted through the optical coupler 26 to the downstream opticalfiber transmission line 4. The second branch light from the opticalcoupler 42 is supplied to an optical band-pass filter 44. The filter 44has a pass band including the wavelengths of the WDM signal light. Thelight passed through the filter 44 is supplied to an O/E converter 46,and an output signal from the O/E converter 46 is supplied to an ALCcircuit (automatic level control circuit) 48 and the SV circuit 22.

[0054] The ALC circuit 48 controls the pumping means 40 so that theoutput level of the O/E converter 46 becomes constant. Morespecifically, in the case that the pump light source included in thepumping means 40 is a laser diode, a drive current (bias current) forthe laser diode is controlled. Of the supervisory informationregenerated in the SV circuit 22, a signal SC giving the number ofchannels is supplied from the SV circuit 22 to the ALC circuit 48. Sincethe ALC circuit 48 performs the above-mentioned feedback control, theoutput level of this optical amplifier is stabilized so as to coincidewith a target level. The target level is to be set so that output powerper channel becomes constant. Accordingly, the target level is to be setaccording to the number of channels of the WDM signal light.

[0055] In this preferred embodiment, the target level is set accordingto the signal SC giving the number of channels. This will now bedescribed more specifically.

[0056]FIG. 3 is a block diagram showing a specific embodiment of the ALCcircuit 48 shown in FIG. 2. The ALC circuit 48 has an operationalamplifier 56 for comparing the output signal (output voltage) from theO/E converter 46 with a reference voltage V_(REF). The output voltagefrom the O/E converter 46 is supplied to a minus input port of theoperational amplifier 56, and the reference voltage V_(REF) is suppliedto a plus input port of the operational amplifier 56. The operationalamplifier 56 outputs a voltage signal corresponding to a leveldifference between the two input ports. This voltage signal is convertedinto a current signal by a V/I converter 58, and the current signal isthen fed back to, for example, the drive current for the pump lightsource in the pumping means 40 (see FIG. 2).

[0057] To set the reference voltage V_(REF) according to the number ofchannels, an MPU (microprocessing unit) 60 is used. The signal SC givingthe number of channels is taken into the MPU 60 through an I/O circuit62, and the reference voltage V_(REF) obtained according to the resultof computation in the MPU 60 is supplied through the I/O circuit 62 tothe operational amplifier 56. The computation in the MPU 60 isperformed, for example, by referring to a data table having the signalSC giving the number of channels as an address. This data table ispreviously stored in a memory 64 connected through the I/O circuit 62 tothe MPU 60. For example, for a signal SC giving a large number ofchannels, a large reference voltage (V_(REF)) is set, whereas for asignal SC giving a small number of channels, a small reference voltage(V_(REF)) is set.

[0058] In this preferred embodiment as described above, the target valuein ALC is set according to the number of channels of the opticaltransmitters 8 being operated in the transmitting station (see FIG. 1).Accordingly, output power per channel from the optical amplifier can bemaintained constant irrespective of a change in the number of channels.Accordingly, by using the optical repeater as shown in FIG. 2, the WDMsystem shown in FIG. 1 can be easily adapted to a change in the numberof WDM channels.

[0059] Referring again to FIG. 2, a further function of the SV circuit22 will be described. The second branch light from the optical coupler36 is supplied to an optical band-pass filter 50 having a pass bandincluding the wavelengths of the WDM signal light. The light output fromthe filter 50 is input to an O/E converter 52, and an output signal fromthe O/E converter 52 is supplied to the SV circuit 22. The output signalfrom the O/E converter 46 on the output side is also supplied to the SVcircuit 22. Accordingly, the levels of input light and output light andthe gain of this optical amplifier can be obtained in the SV circuit 22.By adding such information to the supervisory information transmittedfrom the transmitting station 2, the latter is updated. Then, theupdated supervisory information is transmitted to the downstream opticalrepeater 18 or the receiving station 6. The updated supervisoryinformation is converted into signal light having the wavelength λ_(SV)by an E/O converter 54 incorporated in the SV circuit 22, and thissignal light is then output to the optical fiber transmission line 4 bythe optical coupler 26.

[0060] The updated supervisory information may include a statusinformation indicating the number of channels which determines theoperation of the corresponding optical amplifier. Through the feature ofthe status information, the receiving station 6(shown in FIG. 1) candetect whether or not the corresponding optical amplifier operates basedon the supervisory information sent from the transmitting station 2.This feature of the present invention is very important because thenumber of channels is one of key parameters for operating opticalamplifiers.

[0061]FIG. 3A is a block diagram showing a concrete example of theoptical amplifier shown in FIG. 2. In this example, an EDF(erbium dopedfiber) 39 is used as the optical amplifying medium 38. In order toprevent a resonance in the EDF 39, optical isolators 41A and 41B isconnected to both ends of the EDF 39. The pumping means 40 includes alaser diode 43 outputting pump light and an optical coupler 45 forsupplying the pump light to the EDF 39.

[0062]FIG. 4 is a block diagram of another WDM system to which thepresent invention is applicable. This system includes a transmittingstation 66 for outputting WDM signal light, an optical fibertransmission line 4 for transmitting the WDM signal light output fromthe transmitting station 66, and a receiving station 6 for receiving theWDM signal light transmitted through the optical fiber transmission line4. Two optical repeaters 68 are provided in the optical fibertransmission line 4. Three or more optical repeaters 68 may be providedor one optical repeater 68 may be provided. Each optical repeater 68 hasan optical amplifier 70 and a SV circuit 72 for inputting/outputtingsupervisory information directly from/to the optical amplifier 70.

[0063] The optical transmitter 8 (#5) for mainly transmitting thesupervisory information in the system shown in FIG. 1 is not used inthis preferred embodiment. That is, optical transmitters 8 (#1 to #4)included in the transmitting station 66 can output optical signalsmodulated by transmission data having wavelengths λ₁ to λ₄,respectively. Status signals representing the operational conditions ofthe optical transmitters 8 (#1 to #4) are supplied to a SV circuit 74.The SV circuit 74 generates a tone signal according to the supervisoryinformation including the number of channels of the WDM signal light.This tone signal has a frequency sufficiently lower than that of a mainsignal (transmission data in each optical transmitter 8). The tonesignal is supplied from the SV circuit 74 through a low-pass filter 76to the optical transmitter 8 (#4). The tone signal is superimposed on amodulating signal (transmission data) to be supplied from a terminal 10to the optical transmitter 8 (#4).

[0064] Referring to FIG. 5, there is shown a waveform chart of the tonesignal. A tone signal 80 having a speed sufficiently lower than that ofa main signal 78 is superimposed on one of the WDM optical signals. Thetone signal 80 may be obtained by performing modulation based on thesupervisory information with low-frequency tone, components used as asubcarrier. The frequencies of the tone components are set to 1 KHz to 1MHz, for example, so that each frequency component is not attenuated inthe optical amplifier. In the optical communication art, such atechnique for superimposing the tone signal is known as a subcarrierovermodulation technique.

[0065] Referring to FIG. 6, there is shown a block diagram showing asecond preferred embodiment of the optical repeater. This opticalrepeater may be used as each optical repeater 68 shown in FIG. 4. Thisoptical repeater has a feedback loop including an ALC circuit 48. Thisloop is the same as that shown in FIG. 2, so the description thereofwill be omitted herein. The WDM signal light supplied from the upstreamoptical fiber transmission line 4 is branched into first branch lightand second branch light by an optical coupler 82. The first branch lightfrom the optical coupler 82 is supplied to an optical amplifying medium38. The WDM signal light amplified by the optical amplifying medium 38is transmitted through an optical coupler 42 to the downstream opticalfiber transmission line 4. The second branch light from the opticalcoupler 82 is supplied to an optical band-pass filter 84. The filter 84has a pass band including the wavelengths of the WDM signal light. Thelight output from the filter 84 is supplied to an O/E converter 86. Anoutput signal from the O/E converter 86 is supplied to a band-passfilter 88.

[0066] The filter 88 has a pass band including the carrier frequency ofthe tone signal. Accordingly, the tone signal is extracted by the filter88, and is then supplied to a SV circuit 90. In the SV circuit 90, thesupervisory information is regenerated according to the tone signal, anda signal SC giving the number of channels obtained according to thesupervisory information is supplied from the SV circuit 90 to the ALCcircuit 48. In the ALC circuit 48, a reference voltage V_(REF) (see FIG.3) is set according to the signal SC giving the number of channels.Accordingly, optical output power per channel can be maintained constantirrespective of a change in the number of channels.

[0067] While the tone signal is superimposed on only the optical signalhaving the wavelength λ₄ to be output from the optical transmitter 8(#4) in the system shown in FIG. 4, the tone signal may be superimposedon all the channels of the WDM signal light. In this case, an opticalmodulator is provided between the optical multiplexer 16 and the opticaltransmission line 4 to thereby superimpose the tone signal on the WDMsignal light.

[0068] For example, the number of channels being operated may betransmitted according to the frequency of the tone signal. That is, whenonly one channel is in operation, a tone signal of 10 KHz issuperimposed; when two channels are in operation, a tone signal of 11KHz is superimposed; when three channels are in operation, a tone signalof 12 KHz is superimposed; and so on. Thus, the number of channels isdetected according to the frequency of the tone signal.

[0069] Alternatively, the frequencies of tone signals may be previouslyassigned to all the channels, and the tone signals may be superimposedon the corresponding channels before carrying out wavelength-divisionmultiplexing. In this case, the number of channels being operated can bedetected according to the number of frequency components of the tonesignals.

[0070]FIG. 7 is a block diagram of still another WDM system to which thepresent invention is applicable. This system includes a transmittingstation 92 for outputting WDM signal light, an optical fibertransmission line 4 for transmitting the WDM signal light output fromthe transmitting station 92, and a receiving station 6 for receiving theWDM signal light transmitted through the optical fiber transmission line4. Two optical repeaters 94 are provided in the optical fibertransmission line 4. Three or more optical repeaters 94 may be providedor one optical repeater 94 may be provided. Each optical repeater 94 hasan optical amplifier 96 for amplifying the WDM signal light and an ALCcircuit 100 for controlling the optical amplifier 96 so that the outputlevel of the optical amplifier 96 becomes a target level. A part of theWDM signal light output from the optical amplifier 96 is branched off byan optical coupler 98, and the ALC circuit 100 controls the opticalamplifier 96 so that the power of the branch light from the opticalcoupler 98 becomes constant.

[0071] The transmitting station 92 has five optical transmitters 8 (#1to #5) capable of generating optical signals having differentwavelengths, and an optical multiplexer 16 for wavelength-divisionmultiplexing the optical signals to output WDM signal light. In thispreferred embodiment, the three optical transmitters 8 (#1 to #3) are inoperation, and the other two optical transmitters (#4 and #5) are not inoperation. That is, pulse signals corresponding to main signals aresupplied to drive terminals 10 of the optical transmitters 8 (#1 to #3),and DC biases are supplied to drive terminals 10 of the other opticaltransmitters 8 (#4 and #5). Accordingly, the optical signals to beoutput from the optical transmitters 8 (#1 to #3) are modulated by therespective main signals, and the light to be output from the opticaltransmitters 8 (#4 and #5) is CW light (continuous wave light).

[0072] The reason why the optical transmitters 8 (#4 and #5) not beingoperated are intended to output the CW light having no relation to themain signals is to make constant the total power of the WDM signal lightto be supplied to each optical repeater 94. By thus making the totalpower constant, the target level in the ALC circuit 100 can bemaintained constant in each optical repeater 94 irrespective of thenumber of channels being operated. According to this preferredembodiment, it is therefore unnecessary to change the target level forALC in each optical repeater 94, thereby allowing the ALC circuit to besimplified.

[0073] In the case that the optical amplifier 96 is an EDFA, the subjectto be controlled by the ALC circuit 100 may be set as the power of pumplight to be supplied to an EDF. In the case that the pump light is usedfor control of gain tilt, the subject to be controlled by the ALCcircuit 100 may be set as the attenuation factor of an opticalattenuator (not shown) provided upstream or downstream of the opticalamplifier 96.

[0074]FIG. 8 is a block diagram showing a third preferred embodiment ofthe optical repeater. This optical repeater may be used instead of eachoptical repeater 68 in the WDM system shown in FIG. 4. This opticalrepeater has a feedback loop for ALC. An ALC circuit 102 included inthis loop controls the power of pump light in pumping means 40 so thatthe output level of the optical amplifier becomes a target level. Inthis preferred embodiment, the target level is constant irrespective ofthe number of channels of the WDM signal light.

[0075] To this end, this preferred embodiment employs a compensationlight source 104 for outputting compensation light having a wavelengthincluded in the amplification band but different from the wavelengths ofthe WDM signal light. The first branch light from the optical coupler 82and the compensation light from the light source 104 are combinedtogether in an optical coupler 106, and then supplied to an opticalamplifying medium 38. The second branch light from the optical coupler82 is supplied to an optical band-pass filter 84 having a pass bandincluding the wavelengths of the WDM signal light. The light output fromthe filter 84 is supplied to an O/E converter 86. As previouslydescribed in the system shown in FIG. 4, the output signal from the O/Econverter 86 includes a tone signal modulated by supervisoryinformation. This tone signal is extracted by a band-pass filter 88 andsupplied to a SV circuit 108. The SV circuit 108 controls the power ofthe compensation light to be output from the compensation light source104 according to the number of channels of the WDM signal lightdetermined by regeneration of the supervisory information, therebymaintaining constant the target level in the ALC circuit 102irrespective of the number of channels being operated.

[0076] According to this preferred embodiment, the compensation lightand the WDM signal light are supplied to the optical amplifying medium38. Accordingly, by setting the power of the compensation lightaccording to the number of channels being operated, it is unnecessary tochange the target level in the ALC circuit 102.

[0077]FIG. 9 is a block diagram showing a fourth preferred embodiment ofthe optical repeater. This optical repeater may be used instead of eachoptical repeater 68 in the WDM system shown in FIG. 4. This opticalrepeater has a feedback loop including an ALC circuit 102 similar tothat shown in FIG. 8 and another feedback loop.

[0078] The compensation light from a compensation light source 104 isadded to WDM signal light in an optical coupler 106, and output lightfrom the optical coupler 106 is supplied through an optical coupler 110to an optical amplifying medium 38. In the optical coupler 110, parts ofthe WDM signal light and the compensation light are branched off, andthe resultant branch light is supplied to an optical band-pass filter112. The filter 112 has a pass band including the wavelengths of the WDMsignal light and the wavelength of the compensation light. Output lightfrom the filter 112 is input to an O/E converter 114. A SV circuit 116controls the power of the compensation light so that the output level ofthe O/E converter 114 becomes constant.

[0079] The total power of the WDM signal light and the compensationlight to be supplied to the optical amplifying medium 38 is reflected onthe output level of the O/E converter 114. Accordingly, by providingsuch a feedback loop upstream of the optical amplifying medium 38, thetotal power of the WDM signal light and the compensation light can bemaintained constant. By thus maintaining the total power constant, thetarget level in the ALC circuit 102 can be made constant irrespective ofthe number of channels of the WDM signal light, thereby allowing the ALCcircuit 102 to be simplified. By thus providing the feedback loopincluding the SV circuit 116, this optical repeater in this preferredembodiment need not receive information on the number of channels of theWDM signal light. Accordingly, in the case that the optical repeatershown in FIG. 9 is applied to the system shown in FIG. 4, the SV circuit74 in the transmitting station 66 shown in FIG. 4 may be omitted.

[0080] In the optical repeaters shown in FIGS. 8 and 9, the ALC circuit102 controls the power of the pump light in the pumping means 40.However, in the case that the power of the pump light is used forcontrol of gain tilt, the ALC circuit 102 may control the attenuationfactor of an optical attenuator (not shown) provided upstream ordownstream of the optical amplifying medium 38.

[0081]FIG. 10 is a block diagram showing a first basic configuration ofthe optical amplifier according to the present invention. Like theoptical amplifier included in the optical repeater described above, theoptical amplifier shown in FIG. 10 has an optical amplifying medium 38and pumping means 40. When signal light 130 is supplied to a first end38A of the optical amplifying medium 38 being pumped, amplified signallight 132 is output from a second end 38B of the optical amplifyingmedium 38. In such a condition that the optical amplifying medium 38 isbeing pumped so as to have an amplification band, ASE (amplifiedspontaneous emission) is generated in the optical amplifying medium 38.The ASE is output not only from the second end 38B in the same directionas the propagation direction of the signal light 132, but also from thefirst end 38A in the direction opposite to the propagation direction ofthe signal light 132 as shown by 134. The ASE 134 propagating oppositeto the signal light 132 is extracted by ASE extracting means 136.According to the extracted ASE 134, monitoring means 138 monitorsspectral characteristics giving the wavelength dependence of the powerof the ASE 134. Parameter control means 140 controls a parameter onwhich the gain tilt in the amplification band of the optical amplifyingmedium 38 is dependent (or the gain itself) so that the spectralcharacteristics monitored above are maintained.

[0082] As the optical amplifying medium 38, a doped fiber doped with arare earth element, such as an EDF, may be used. Alternatively, asemiconductor chip may be used (semiconductor optical amplifier). In thelatter case, the pumping means 40 includes means for injecting a currentinto the chip. Specifically, a pumping voltage is applied across a pairof electrodes of the semiconductor optical amplifier. The pumping means40 suitable for the doped fiber includes a pump light source foroutputting pump light, and optical coupling means operatively connectedto at least one of the first end 38A and the second end 38B of theoptical amplifying medium 38 to supply the pump light to the opticalamplifying medium 38.

[0083] In this specification, the wording that optical components areoperatively connected to each other includes the case that the opticalcomponents are directly connected together by fiber connection orspatial connection using a collimated beam, and further includes thecase that the optical components are connected through another opticalcomponent such as an optical filter.

[0084] In the case that the pumping means 40 includes the pump lightsource, the power of the pump light may be adopted as the parameter tobe controlled by the parameter control means 140. In this case, the pumplight source cannot be included in a feedback loop of ALC for makingconstant the power of the amplified signal light 132 (total gain of theoptical amplifier). Therefore, in performing the ALC, a feedback loopincluding an optical attenuator having a variable attenuation factor maybe provided.

[0085] In the case that this optical amplifier includes a compensationlight source 142 for supplying to the optical amplifying medium 38compensation light having a wavelength included in the amplificationband of the optical amplifying medium 38, the parameter to be controlledby the parameter control means 140 may be the power of the compensationlight. In this case, the pump light source can be included in thefeedback loop for the ALC. The wavelength of the compensation light isset different from the wavelength of the signal light.

[0086] In the case that this optical amplifier is applied to a WDMsystem, WDM signal light is supplied into the optical amplifying medium38 from the first end 38A.

[0087] Gain characteristics of the optical amplifying medium 38, i.e.,the gain tilt, are reflected on the spectral characteristics of the ASE134. Since the ASE 134 propagates opposite to the signal light in theoptical amplifying medium 38, the spectral characteristics of the ASE134 are not influenced by the number of channels of WDM signal light,input level, and accumulated ASE in principle. Accordingly, bycontrolling the parameter on which the gain tilt depends so that thespectral characteristics of the ASE 134 are maintained, a constant gaintilt can be easily obtained. Specific embodiments of a monitoring methodfor the spectral characteristics will be hereinafter described.

[0088] Preferably, the first basic configuration of the opticalamplifier shown in FIG. 10 has an optical band-pass filter 143operatively connected to the second end 38B of the optical amplifyingmedium 38. The effectiveness of the filter 143 will now be described.

[0089] The spectral characteristics of forward ASE propagating in thesame direction as the propagation direction of the signal light in theoptical amplifying medium 38 are influenced by the input level of thesignal light and accumulated ASE. To the contrary, the spectralcharacteristics of backward ASE propagating in the direction opposite tothe propagation direction of the signal light in the optical amplifyingmedium 38 are not influenced by these factors in principle. However, inactual, if there is any little reflection on the output side of theoptical amplifying medium 38, there is a possibility that accumulatedASE may be reflected, and this reflected accumulated ASE may then beamplified in the optical amplifying medium 38 to mix into the backwardASE. Accordingly, in the case that such mixing of the accumulated ASEinto the backward ASE becomes a problem, the optical band-pass filter143 having a proper pass band is used.

[0090] Referring to FIG. 11A, there is shown a preferable pass band ofthe optical band-pass filter 143. The shortest wavelength λ_(L) in thepass band is set slightly shorter than the shortest wavelength of theWDM signal light, and the longest wavelength λ_(H) in the pass band isset slightly longer than the longest wavelength of the WDM signal light.With this setting, the power of accumulated ASE can be effectivelyreduced.

[0091] Preferably, the monitoring means 138 shown in FIG. 10 has twooptical band-pass filters having different pass bands (e.g., opticalband-pass filters 170 and 172 of a spectrum monitor shown in FIG. 14).In this case, as shown in FIG. 11B, the shortest wavelength and thelongest wavelength in the pass band of one of the two filters are set toλ_(L)−Δλ and λ_(L), respectively, whereas the shortest wavelength andthe longest wavelength in the pass band of the other filter are set toλ_(H) and λ_(H)+Δλ, respectively. With this setting, even if a reflectedcomponent of the accumulated ASE is mixed into the backward ASE, noinfluence of such mixing appears to a result of monitoring.

[0092]FIG. 12 is a block diagram showing a first preferred embodiment ofthe optical amplifier according to the present invention. Signal lightto be amplified is supplied through an optical coupler 144 into anoptical amplifying medium 38 from its first end 38A. The ASE propagatingin the direction opposite to the signal light in the optical amplifyingmedium 38 is extracted by the optical coupler 144. The extracted ASE issupplied to a spectrum monitor 146. As the optical coupler 144, a fiberfused type of optical coupler, a WDM coupler that is a special form ofthis type of optical coupler, or an optical circulator may be used. Alaser diode 148 as a pump light source is used to pump the opticalamplifying medium (e.g., doped fiber) 38. Pump light output from thelaser diode 148 is supplied to the optical amplifying medium 38 throughan optical coupler 150 connected to a second end 38B of the opticalamplifying medium 38. The laser diode 148 is supplied with a biascurrent from a drive circuit 152. The power of the pump light can becontrolled according to the bias current.

[0093] The ASE spectral characteristics monitored by the spectrummonitor 146 is supplied to a control circuit 154. The control circuit154 controls the bias current to be supplied from the drive circuit 152to the laser diode 148 so that the spectral characteristics from thespectrum monitor 146 are maintained.

[0094] In this preferred embodiment, the bias current for the laserdiode 148 for outputting the pump light is included in a feedback loopfor maintaining the gain tilt. Accordingly, ALC cannot be performed byusing the bias current for the laser diode 148. To perform ALC, theamplified signal light output from the second end 38B of the opticalamplifying medium 38 through the optical coupler 150 is input into anoptical attenuator 156. The attenuation factor of the optical attenuator156 is variable. The light output from the optical attenuator 156 isbranched into first branch light and second branch light by an opticalcoupler 158. The first branch light from the optical coupler 158 isoutput to an optical transmission line (not shown). The second branchlight from the optical coupler 158 is supplied to an optical band-passfilter 160 having a pass band including the wavelength of the signallight. Output light from the filter 160 is converted into an electricalsignal by an O/E converter 162. An ALC circuit 164 controls theattenuation factor of the optical attenuator 156 so that the outputlevel of the O/E converter 162 becomes constant.

[0095]FIG. 13 is a graph for illustrating an example of the gain tilt inthe optical amplifier shown in FIG. 12. There are shown in FIG. 13 thespectra of output beams when WDM signal beams each having four channelsof wavelengths of 1548 nm, 1551 nm, 1554 nm, and 1557 nm are input withthe same input power (−35 dBm/ch) into an EDF being pumped. In FIG. 13,the vertical axis represents output power (dBm) and the horizontal axisrepresents wavelength (nm). The spectrum shown by A corresponds to thecase where the power of pump light is relatively large. In this case, anegative gain tilt occurs. That is, the differential of gain withrespect to wavelength is negative (dG/dλ<0). The spectrum shown by Ccorresponds to the case where the power of pump light is relativelysmall. In this case, a positive gain tilt is obtained (dG/dλ>0). Thespectrum shown by B corresponds to the case where the power of pumplight is optimum such that no gain tilt occurs. In this case, thedifferential of gain with respect to wavelength is 0 (dG/dλ=0). Eachspectrum shown in FIG. 13 has such a shape that four sharp spectracorresponding to the four channels of each WDM signal light aresuperimposed on a spectrum of ASE.

[0096] In the optical amplifier shown in FIG. 12, the ASE output fromthe first end 38A of the optical amplifying medium 38 is extracted, sothat the spectrum of the WDM signal light is not superimposed on thespectrum of the ASE. Accordingly, the spectrum monitor 146 can monitorthe ASE spectrum with a high accuracy without the influence of the powerof the WDM signal light.

[0097] Such ASE propagating in the direction opposite to signal lightwill be hereinafter referred to as backward ASE. Letting P_(ASE) (λ)denote the power of backward ASE, it is given by Eq. (1), which is afunction of wavelength λ.

P _(ASE)(λ₀)=2n_(SP)(λ₀)h(C/λ ₀) [G(λ₀)−1]Δλ  (1)

[0098] where n_(SP)(λ₀O) is the spontaneous emission factor, h is thePlanck constant, C is the velocity of light in a vacuum, λ₀ is thecenter wavelength in a band to be monitored, and Δλ is the bandwidth ofthe band to be monitored. Usually, the wavelength dependence of eachparameter is substantially constant in the range of Δλ, so that Δ₀ isused as a representative. The spontaneous emission factor n_(SP)(λ₀) haswavelength dependence, and a method of improving a monitoring accuracyso as to cope with this wavelength dependence will be hereinafterdescribed.

[0099] In Eq. (1), G(λ₀) represents the gain to be given as a functionof wavelength. In this manner, gain characteristics (wavelengthdependence of gain) are reflected in the spectrum of backward ASE.Accordingly, gain characteristics can be evaluated by cutting out two ormore narrow bands included in an amplification band, individuallydetecting the powers in these narrow bands, and obtaining a deviationbetween detected values. Specifically, the spectrum monitor 146 shown inFIG. 12 includes means for branching backward ASE into first branchlight and second branch light, a first optical band-pass filter having afirst narrow pass band included in an amplification band, for receivingthe first branch light, a second optical band-pass filter having asecond narrow pass band included in the amplification band but differentfrom the first pass band, for receiving the second branch light, firstand second photodetectors for respectively receiving lights passedthrough the first and second optical band-pass filters, and means fordetecting a deviation between output levels of the first and secondphotodetectors. This configuration will now be described morespecifically.

[0100]FIG. 14 is a block diagram showing a preferred embodiment of thespectrum monitor 146 shown in FIG. 12. The backward ASE generated fromthe optical amplifying medium 38 (see FIG. 12) is supplied through anoptical isolator 166 to an optical coupler 168. If reflection from abackward ASE monitoring system is low, the optical isolator 166 isunnecessary. The optical coupler 168 branches the input backward ASEinto first branch beam and second branch beam. A branching ratio betweenthe first branch beam and the second branch beam is set to 1:1, forexample. The first and second branch beams are respectively supplied tooptical band-pass filters 170 and 172. In the case that the backward ASEhas a spectrum similar to the ASE spectrum as shown in FIG. 13, thecenter wavelengths in the pass bands of the filters 170 and 172 arerespectively set to 1541 nm and 1559 nm, for example. The beams passedthrough the filters 170 and 172 are respectively supplied to photodiodes174 and 176. Since output signals from the photodiodes 174 and 176 arecurrent signals, I/V converters (current/voltage converters) 178 and 180respectively corresponding to the photodiodes 174 and 176 are used.Output voltage signals from the I/V converters 178 and 180 arerespectively supplied to a minus input port and a plus input port of anoperational amplifier 182. As a result, an output signal from theoperational amplifier 182 reflects a deviation between the output levelsof the photodiodes 174 and 176.

[0101] Accordingly, by feeding back the output signal from theoperational amplifier 182 to the bias current for the laser diode 148(see FIG. 12), the spectral characteristics of the backward ASEgenerated in the optical amplifying medium 38 can be maintained, so thatthe gain tilt can be maintained constant. By suitably setting a targetvalue of the deviation in the feedback loop, the gain tilt can be madeflat as shown by B in FIG. 13, for example.

[0102] Referring to FIG. 15, there is shown another spectrum monitorapplicable to the present invention. In this preferred embodiment,output signals from I/V converters 178 and 180 are taken through an I/Oport 184 into an MPU (microprocessing unit) 186. The MPU 186 isconnected through the I/O port 184 to a memory 188. The MPU 186 receivesthe output levels of the converters 178 and 180, calculates a deviationbetween the output levels, and outputs the calculated deviation throughthe I/O port 184.

[0103] As described above, the spontaneous emission factor n_(SP)(λ₀) inEq. (1) has wavelength dependence, that is, depends on the wavelength λ₀to be monitored. Accordingly, in the case that higher monitoringaccuracies are required, a data table of the spontaneous emission factorn_(SP)(λ₀) using wavelength as a parameter may be previously stored inthe memory 188 to obtain accurate spectral characteristics according tothe data table. For example, the gain G(λ) can be accurately calculatedaccording to a calculated value of the deviation.

[0104] In the spectrum monitor shown in FIG. 14 or FIG. 15, the twonarrow bands (first and second pass bands) are cut out from theamplification band. However, three or more optical band-pass filters maybe used to cut out three or more narrow bands corresponding to thenumber of the optical band-pass filters from the amplification band. Inthis case, the accuracy of monitoring of the backward ASE to becalculated by the MPU 186 can be improved, for example.

[0105]FIG. 15A is a block diagram showing a concrete example of theoptical amplifier shown in FIG. 12. In this example, an EDF(erbium dopedfiber) 39 is used as the optical amplifying medium 38. In order toprevent a resonance in the EDF 39, an optical isolator 145A is providedon the input side of the optical coupler 144, and another opticalisolator 145B is provided between the optical coupler 150 and theoptical attenuator 156. The pump light from the laser diode 148propagates in the direction opposite to the signal light in the EDF 39.That is, backward pumping is adopted. Alternatively, forward pumping maybe adopted such that the pumping light propagates in the same directionas the signal light. Further, pump light beams may be supplied into theEDF 39 from both ends thereof, thereby improving a pumping efficiency.

[0106]FIG. 15B is a block diagram showing an optical amplifier to whichthe forward pumping is adopted. In place of the optical coupler150(shown in FIG. 15A), an optical coupler 150′ is provided on the inputside of the optical isolator 145A. The pump light from the laser diode148 and the signal light to be amplified are supplied to the EDF 39through the optical coupler 150′, the optical isolator 145A, and theoptical coupler 144.

[0107]FIG. 16 is a block diagram showing a second preferred embodimentof the optical amplifier according to the present invention. In contrastwith the optical amplifier shown in FIG. 12, the optical amplifier shownin FIG. 16 is characterized in that an optical attenuator 156′ for ALCis provided upstream of the optical amplifying medium 38. That is,signal light to be input into the optical amplifying medium 38 from itsfirst end 38A is preliminarily attenuated rather than attenuatingamplified signal light. The attenuation factor of the optical attenuator156′ is controlled by an ALC circuit 164 so that an output level of an0/E converter 162 corresponding to an output level of this opticalamplifier becomes constant.

[0108] According to the optical amplifier shown in FIG. 12 or FIG. 16, aconstant gain tilt can be maintained, and ALC can also be performed.

[0109]FIG. 17 is a block diagram showing a third preferred embodiment ofthe optical amplifier according to the present invention. This opticalamplifier employs a compensation light source for supplying compensationlight to the optical amplifying medium 38. The power of the compensationlight is controlled so that the spectral characteristics of backward ASEare maintained. With this change, the power of pump light is subjectedto ALC.

[0110] A laser diode 190 is used as the compensation light source. Thecompensation light from the laser diode 190 is supplied through anoptical coupler 192 to the optical amplifying medium 38 from its firstend 38A. Signal light to be amplified is supplied through an opticalcoupler 144 for extracting the backward ASE and the optical coupler 192for the compensation light in this order to the optical amplifyingmedium 38 from its first end 38A. The backward ASE generated in theoptical amplifying medium 38 is supplied through the optical coupler 192and the optical coupler 144 in this order to a spectrum monitor 146. Thelaser diode 190 is supplied with a bias current from a drive circuit194. The bias current to be supplied to the laser diode 190 iscontrolled by a control circuit 154. The control circuit 154 controlsthe bias current for the laser diode 190 so that the spectralcharacteristics of the backward ASE monitored by the spectrum monitor146 are maintained. Accordingly, the power of the compensation light tobe output from the laser diode 190 is controlled to maintain constantthe gain characteristics of this optical amplifier.

[0111] In this preferred embodiment, the power of pump light is not usedin the control for maintaining the gain characteristics constant.Accordingly, a pump light source can be included in the feedback loopfor ALC. Since the compensation light is used for maintenance of thespectral characteristics, the compensation light is removed by anoptical filter 160, and output light from the optical filter 160 isconverted into an electrical signal by an O/E converter 162. A laserdiode 148 as the pump light source is supplied with a bias current froma drive circuit 152. The bias current is controlled by an ALC circuit164. Also according to the third preferred embodiment, a constant gaintilt can be maintained, and ALC can be performed. Further, an opticalattenuator for ALC is unnecessary in the third preferred embodiment.

[0112]FIG. 18 is a block diagram showing a fourth preferred embodimentof the optical amplifier according to the present invention. In thispreferred embodiment, reflecting mirrors 196 and 198 are operativelyconnected to the second end 38B of the optical amplifying medium 38, soas to improve the accuracy of monitoring of the spectral characteristicsof backward ASE in the spectrum monitor 146. In the case that thespectrum monitor 146 is configured as shown in FIG. 14, the reflectingmirror 196 reflects light having a wavelength included in the pass bandof the optical band-pass filter 170 and transmits other light, and thereflecting mirror 198 reflects light having a wavelength included in thepass band of the optical band-pass filter 172 and transmits other light.By providing the reflecting mirrors 196 and 198, a part of the forwardASE output from the second end 38B of the optical amplifying medium 28falling within a specific band can be reciprocated in the opticalamplifying medium 38. Accordingly, the input powers into the photodiodes174 and 176 shown in FIG. 14 can be increased to thereby improve thespectrum monitoring accuracy. In the case that three or more narrowbands are cut out from the ASE spectrum, three or more reflectingmirrors corresponding to the number of the narrow bands are used.

[0113] In a doped fiber such as an EDF, SE (spontaneous emission) leakssideways. The gain characteristics in the doped fiber are reflected inthe SE. Further, the SE leaking sideways is not influenced by the numberof channels of WDM signal light, input level, and accumulated ASE. Thisfact has been reported by Aida et al. in the International Conference,1991 (Optical Amplifiers and their Applications; OAA, FE3), in which ithas become apparent that gain G(λ) can be obtained from an integralP_(SE)(λ) of SE from a side surface of a doped fiber over the fiberlength L in accordance with Eqs. (2) and (3).

P _(SE)(λ)={1n[G(λ)]+α_(S)(λ)L}/C(λ)  (2)

C(λ)=η(λ) {σ_(e)(λ) +σ_(a)(λ)}τ/{h(C/λ)π(γ_(Er))²}  (3)

[0114] where σ_(e)(λ), σ_(a)(λ), and α_(S)(λ) are the emission crosssection at λ, the absorption cross section at λ, and the loss at λ,respectively, and τ and γ_(Er), are the spontaneous emission lifetimeand the radius of a region doped with Er, respectively. Further, η(λ) isthe coefficient having wavelength dependence. Accordingly, by monitoringthe spectral characteristics of SE leaking sideways, gaincharacteristics (gain tilt) can be grasped.

[0115]FIG. 19 is a block diagram showing a second basic configuration ofthe optical amplifier according to the present invention. An opticalamplifying medium 38 has an optical waveguide structure to which signallight is supplied. Pumping means 40 pumps the optical amplifying medium38 so that the optical amplifying medium 38 has an amplification bandincluding the wavelength of the signal light. SE extracting means 200extracts SE light leaking sideways from the optical waveguide structureof the optical amplifying medium 38. Monitoring means 138 monitorsspectral characteristics giving the wavelength dependence of the powerof the extracted SE. Parameter control means 140 controls a parameter onwhich the gain tilt in the amplification band of the optical amplifyingmedium 38 depends (or gain itself) so that the spectral characteristicsmonitored are maintained.

[0116] In the configuration shown in FIG. 19, the parameter to becontrolled by the parameter control means 140 is the power of pump lightin the pumping means 40. Alternatively, the parameter to be controlledby the parameter control means 140 may be the power of compensationlight like in FIG. 10.

[0117]FIG. 20 is a block diagram of a spectrum monitor that can be usedas the SE extracting means 200 and the monitoring means 138 shown inFIG. 19. An EDF 202 is used as the optical amplifying medium 38 (seeFIG. 19). Means for pumping the EDF 202 is not shown. The EDF 202 isaccommodated in a case 204 such as an integrating sphere configured soas to block entering of external light. A cover of the EDF 202 ispartially removed, and SE leaks sideways from an uncovered portion ofthe EDF 202. The SE is supplied to optical band-pass filters 206 and208. The filters 206 and 208 have their pass bands similar to the passbands of the optical band-pass filters 170 and 172 shown in FIG. 15,respectively.

[0118] The beams passed through the filters 206 and 208 are convertedinto current signals by photodiodes 210 and 212, respectively. Thecurrent signals from the photodiodes 210 and 212 are next converted intovoltage signals by I/V converters 214 and 216, respectively. The voltagesignals from the I/V converters 214 and 216 are supplied to anoperational amplifier 218. As described above, the gain characteristicsof the optical amplifier are reflected on the SE spectrum. Accordingly,by controlling the power of pump light according to the SE spectralcharacteristics monitored, the gain tilt of the optical amplifier can bemaintained constant.

[0119] The spectrum monitor shown in FIG. 20 may be modified accordingto the configuration shown in FIG. 15. That is, an MPU is used formonitoring of the spectral characteristics. In this case, by previouslystoring C(λ), α_(S), and L in Eqs. (2) and (3) in a memory, themonitoring accuracy of the spectral characteristics can be improved.Further, three or more narrow bands may be cut out from the SE spectrum,and optical power in each band may be detected to perform variouscontrols according to the result of detection.

[0120]FIG. 21 is a block diagram showing a third basic configuration ofthe optical amplifier according to the present invention. This opticalamplifier is configured by cascading a first optical amplifier unit 220and a second optical amplifier unit 222. Each of the optical amplifierunits 220 and 222 has the first basic configuration shown in FIG. 10.Light amplified in the first optical amplifier unit 220 is attenuated byan optical attenuator 224 having a variable attenuation factor, and nexttransmitted through a dispersion compensating fiber (DCF) 226 to thesecond optical amplifier unit 222. The DCF 226 has a dispersion value soas to cancel chromatic dispersion influenced the signal light in atransmission line. The light output from the second optical amplifierunit 222 is branched into first branch light and second branch light byan optical coupler 228. The first branch light from the optical coupler228 is output to an optical transmission line (not shown). The secondbranch light from the optical coupler 228 is converted into anelectrical signal by an O/E converter 230. An ALC circuit 232 controlsthe attenuation factor of the optical attenuator 224 so that the outputlevel of the O/E converter 230 is maintained constant.

[0121] Such a two-stage configuration of the optical amplifier in thispreferred embodiment is due to the following reasons. The first reasonis that a loss in a DCF is large in general, and it is thereforenecessary to raise the level of the signal light to some extent on theupstream side of the DCF 226. The second reason is that if an opticalamplifier gain on the upstream side of the DCF 226 is made excess toincrease the power of the signal light, nonlinear effects are prone tooccur in the DCF 226. If four-wave mixing (FWM) as one of the nonlineareffects occurs in the DCF 226 in a system employing WDM, interchannelcrosstalk is increased. Further, self-phase modulation (SPM) alsoinvites a deterioration in signal quality.

[0122] According to the third basic configuration, a constant gain tiltcan be maintained, and ALC can also performed.

[0123] As described above, according to an aspect of the presentinvention, it is possible to provide an optical communication systemwhich can easily respond to a change in the number of WDM channels.According to another aspect of the present invention, it is possible toprovide an optical amplifier which can maintain a constant gain tilt.According to a further aspect of the present invention, it is possibleto provide an optical amplifier which can maintain a constant gain tiltand allows automatic level control.

[0124] The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. An optical amplifier comprising: a first detectordetecting a supervisor signal in wavelength multiplexed light; a seconddetector detecting an input light power to the optical amplifier; anoptical amplifying medium amplifying the input light; a third detectordetecting an output power of the optical amplifying medium; and acontroller controlling a gain of the optical amplifying medium inaccordance with the supervisor signal from the first detector, the inputlight power from the second detector and the output power of the opticalamplifying medium from the third detector.
 2. An optical amplifiercomprising: an optical amplifying medium amplifying a wavelengthdivision multiplexed (WDM) light; and a controller controlling a gain ofthe optical amplifying medium in accordance with a supervisory signal inthe WDM light detected by the optical amplifier, a power of the WDMlight input to the optical amplifying medium as detected by the opticalamplifier, and an output power of the amplified WDM light from theoptical amplifier medium as detected by the optical amplifier.
 3. Anoptical amplifier comprising: an optical amplifying medium amplifying awavelength division multiplexed (WDM) light; and means for controlling again of the optical amplifying medium in accordance with a supervisorysignal in the WDM light detected by the optical amplifier, a power ofthe WDM light input to the optical amplifying medium as detected by theoptical amplifier, and an output power of the amplified WDM light fromthe optical amplifier medium as detected by the optical amplifier.